1. Field of the Invention
This invention relates to the hydrotreating of heavy hydrocarbon oil fractions to produce a product containing lighter hydrocarbon components with lower metals content.
2. Background of the Art
It is well known to hydrotreat heavy hydrocarbon oils such as petroleum residue to convert such heavy oils to lighter products and to remove contaminants such as metals, sulfur and nitrogen. Hydrotreating is generally performed in the presence of a catalyst. The reactor may contain the catalyst in a fixed or fluidized bed, or, alternatively, the catalyst may be dispersed in the heavy oil and fed along with the feedstock through a reactor which provides the appropriate residence time at a temperature necessary for the desired conversion.
Metal contaminants may deposit on the catalyst of a fixed bed and deactivate it after a period of time thereby requiring shutdown of the reactor and replacement or regeneration of the catalyst. For treatment of heavy oils containing a high content of contaminant metals a process based on dispersed catalyst may be preferred. The dispersed catalyst, which may be a metal organic compound, must be continuously added to the feed.
A difficulty in employing catalytic hydrotreating at moderate hydrogen pressure (i.e., less than about 2500 psig) is that efforts to achieve high conversion can result in the formation of excessive amounts of coke (i.e., greater than about 1 to 2 wt. %) which can deposit inside the reactor and impair continuous operation of the unit. Operating the reactor at lower temperatures (i.e., less than about 750.degree. F.) can reduce coking but achieves low conversion and requires very long residence times. Higher operating temperatures (i.e., greater than about 820.degree. F.) permit shorter residence times but yield too much coke at high conversion.
Crude petroleum oils, as well as the heavier hydrocarbon fractions derived therefrom, generally contain metallic contaminants which have an adverse effect on catalysts utilized in various processes to which the crude oil, or heavy hydrocarbon fraction thereof, is ultimately subjected. The most common metallic contaminants are nickel and vanadium, although other metals including iron, copper, etc., are often present. These metals occur in a variety of forms. They may exist as metal oxides or sulfides introduced into the crude oil as metallic scale or similar particles, or they may be present in the form of soluble salts of such metals. Usually, however, they exist in the form of stable organometallic compounds such as metal porphyrins and the various derivatives thereof. Although the metallic contaminants existing as oxide or sulfide scale may be removed, at least in part, by relatively simple filtering techniques, and the water-soluble salts are, at least in part, removable by washing and subsequent dehydration, a more severe treatment is required to remove the stable organometallic compounds, such as metal porphyrins, before the crude oil or heavy hydrocarbon fraction thereof is suitable for further processing. Notwithstanding that the concentration of these organometallic compounds is relatively small, for example, often less than about 10 ppm calculated as the elemental metal, subsequent processing techniques are adversely affected thereby. For example, when a hydrocarbon charge stock containing metals in excess of about 3.0 ppm is subjected to catalytic cracking, the metals become deposited upon the catalyst, altering the composition thereof to the extent that undesirable by-products are formed. That is to say, the composition of the catalyst composite, which is closely controlled with respect to the nature of the charge stock being processed and the quality and quantity of the product desired, is considerably changed as a result of the metal deposition thereon during the course of the cracking process. As a consequence, the liquid product recovery is reduced, and coke and hydrogen are formed in excessive amounts, the former producing relatively rapid catalyst deactivation. The presence of stable organometallic compounds, including metal porphyrins, adversely effects other processes including catalytic reforming, isomerization, hydrodealkylation, etc.
In addition, crude petroleum oils, and the heavier hydrocarbon fractions thereof, generally contain undesirable nitrogenous and sulfurous compounds which may be removed from the petroleum oil by hydrotreating wherein these compounds are converted respectively to ammonia and hydrogen sulfide which are readily separated from the system in a gaseous phase. However, reduction in the concentration of the stable organometallic compounds, to the extent that the crude oil or heavy hydrocarbon fraction thereof becomes suitable for further processing, is not as readily achieved.
The crude oils and other heavy hydrocarbon fractions generally contain considerable quantities of pentane-insoluble materials present in the form of a colloidal suspension or dispersion. These pentane-insoluble materials, described as asphaltenes, are a carbonaceous material considered as coke precursors having a tendency to become deposited as a gummy hydrocarbonaceous residue. The asphaltenes contain the bulk of the difficult to remove metal contaminants.